Molecular tracking of individual host use in the Shiny Cowbird – a generalist brood parasite

Generalist parasites exploit multiple host species at the population level, but the individual parasite's strategy may be either itself a generalist or a specialist pattern of host species use. Here, we studied the relationship between host availability and host use in the individual parasitism patterns of the Shiny Cowbird Molothrus bonariensis, a generalist avian obligate brood parasite that parasitizes an extreme range of hosts. Using five microsatellite markers and an 1120-bp fragment of the mtDNA control region, we reconstructed full-sibling groups from 359 cowbird eggs and chicks found in nests of the two most frequent hosts in our study area, the Chalk-browed Mockingbird Mimus saturninus and the House Wren Troglodytes aedon. We were able to infer the laying behavior of 17 different females a posteriori and found that they were mostly faithful to a particular laying area and host species along the entire reproductive season and did not avoid using previously parasitized nests (multiple parasitism) even when other nests were available for parasitism. Moreover, we found females using the same host nest more than once (repeated parasitism), which had not been previously reported for this species. We also found few females parasitizing more than one host species. The use of an alternative host was not related to the main hosts' nest availability. Overall, female shiny cowbirds use a spatially structured and host species specific approach for parasitism, but they do so nonexclusively, resulting in both detectable levels of multiple parasitism and generalism at the level of individual parasites.Fil: de la Colina, María Alicia. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; ArgentinaFil: Hauber, Mark E.. City University of New York; Estados UnidosFil: Strausberger, Bill M.. Field Museum of Natural History; Estados UnidosFil: Reboreda, Juan Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; ArgentinaFil: Mahler, Bettina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Ecología, Genética y Evolución de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Ecología, Genética y Evolución de Buenos Aires; Argentin

Developmental asynchrony and host species identity predict variability in nestling growth of an obligate brood parasite: a test of the “growth-tuning” hypothesis

Generalist obligate brood parasites are excellent models for studies of developmental plasticity, as they experience a range of social and environmental variation when raised by one of their many hosts. Parasitic Brown-headed Cowbirds (Molothrus ater (Boddaert, 1783)) exhibit host-specific growth rates, yet cowbird growth rates are not predicted by hosts’ incubation or brooding periods. We tested the novel “growth-tuning” hypothesis which predicts that total asynchrony between cowbirds’ and hosts’ nesting periods results in faster parasitic growth in nests where host young fledge earlier than cowbirds. We tested this prediction using previously-published and newly-added nestling mass data across diverse host species. Total nesting period asynchrony (summed across incubation and brooding stages) predicted cowbird growth; 8-day old cowbirds were heavier in host nests with relatively shorter nesting periods. We further explored the drivers of variation in growth using mass measurements of cowbirds in Song Sparrow (Melospiza melodia (Wilson, 1810)) and Red-winged Blackbird (Agelaius phoeniceus (Linnaeus, 1766)) nests. Our top models included host species (cowbirds grew faster in sparrow nests), numbers of nestmates (slowest when raised alone), and sex (males grew faster). These results confirm that multiple social and environmental factors predict directional patterns of developmental plasticity in avian generalist brood parasites.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Parasitic cowbirds may defeat host defense by causing rejecters to misimprint on cowbird eggs

Experiments suggest that some hosts that reject parasitic brown-headed cowbird (Molothrus ater) eggs develop egg recognition by imprinting on their own eggs, probably at the first nest naïve birds attend. This raises the possibility that parasitism of naïve individuals might cause them to mistakenly misimprint on, and accept, cowbird eggs. In an egg-learning experiment designed to cause misimprinting, hosts accepted cowbird eggs when they had laid only 1 egg even though most rejected cowbird eggs when nests had ≥2 host eggs. These findings show that cowbirds can cause misimprinting by parasitizing naïve hosts. If done opportunistically, misimprinting should become more likely as cowbird abundance increases. In accord with this expectation, simulated brood parasitism showed that 3 host species that usually reject cowbird eggs were more tolerant of cowbird eggs in the Great Plains, where cowbirds reach maximal historic abundance, than in eastern North America where cowbirds are less common. This relation between parasite abundance and host responses is opposite to that found in cuckoo hosts. In addition, these 3 rejecter species had high rates (10.8--30.4%) of natural cowbird parasitism at an Illinois site where cowbirds are extremely, and historically, abundant. These rejecters accepted most cases of researcher-detected parasitism, even though they rarely accept where cowbirds are less common. Despite the potentially high cost resulting from egg rejection, we demonstrate that parasitism of "rejecters" can be adaptive for cowbirds particularly when host egg recognition involves learning. Copyright 2009, Oxford University Press.

Indirect estimates of breeding and natal philopatry in an obligate avian brood parasite

In theory, obligate brood parasitic birds are freed from several of the temporal and spatial constraints of parental care for dependent young. Yet, similar to parental bird species, adults in several avian brood parasites show a territorial spacing system while breeding, including site fidelity within and across years. Banding-based capture and sighting studies are also suggestive of non-parasite-like lower levels of natal philopatry in avian brood parasites. We analyzed the potential correlation of physical distance with genetic structure of a Brown-headed Cowbird Molothrus ater population, sampling parasitic chicks from nests across different temporal and spatial scales of its common host, the Eastern Phoebe Sayornis phoebe, near Ithaca, New York, USA. In support of extensive breeding but not historical patterns of natal philopatry, we detected no significant covariation of genetic similarity by distance of nestling parasites at the scale beyond that of individual host nest sites. These results contribute towards a baseline for future behavioral and genetic comparisons of whether and how parasitic versus parental reproductive strategies impact patterns of avian population genetic structure across space and time.Mark E. Hauber, Bill M. Strausberger, Kevin A. Feldheim, Justin Lock, Phillip Casse